The MPEG-2 standard addresses the combining of one or more elementary streams of video, audio and other data into single or multiple streams that are suitable for storage or transmission. In very general terms, the MPEG-2 standard for transmitting digital video and associated audio and other information involves the following three steps. In the first step, a digital video signal (from a digital camera or from an analog to digital converter) is compressed by analyzing and encoding the signal using spatial and temporal redundancy. Spatial redundancy refers to the redundant information inside one video frame while temporal redundancy refers to the redundant information between consecutive frames. This process generates: Intra-frames (I-frames), which contain all of the information in an entire image; Predicted frames (P-Frames), which have some compression as they are predicted based on past I-frames and/or other P-frames; and Bi-directionally predicted frames (B-frames), which are the most compressed images as they are predicted from past and future I-Frames and P-Frames. In the second step carried out concurrently with the first step, an audio signal is compressed by removing low-power tones adjacent high-power tones. Removal of these tones does not affect the signal, because the high-power tones tend to mask the lower-power tones, making them inaudible to the human ear. In the final third step, the compressed video signals, audio signals and related time stamps of those signals are assembled into packets and inserted into a Packetized Elementary Stream (PES). Each packet in a packetized elementary stream contains overhead information such as a start code, stream ID, packet length, optional packetized elementary stream header and stuffing bytes, in addition to the actual packet bytes of video and audio data.
To facilitate the multiplexing together of several streams of packetized elementary streams of different types of data, a Programme Specific Information (PSI) table is also created, which includes a series of tables to reassemble specific packetized elementary stream within multiple channels of packetized elementary streams. The packetized elementary stream and the program specific information provide the basis for a Transport Stream (TS) of packetized elementary stream and program specific information packets.
Of particular interest to the invention disclosed herein is the transport stream as defined in Annex D of the “ATSC Digital Television Standard” published by the Advanced Television Systems Committee (ATSC) in 1995 as its document A/53. This standard defines the broadcasting of digital television (DTV) signals within the United States of America and is referred to in this specification simply as “A/53”. Annex D of A/53 specifies that the original data transport stream is composed of 187-byte packets of data corresponding to MPEG-2 packets without their initial sync bytes. Annex D of A/53 specifies that data are to be randomized by being exclusive-ORed with a specific 216-bit maximal length pseudo-random binary sequence (PRBS) which is initialized at the beginning of each data field. Annex D of A/53 specifies (207, 187) Reed-Solomon forward-error-correction (R-S FEC) coding of packets of randomized data followed by convolutional interleaving. The convolutional interleaving prescribed by A/53 provides error correction capability for continuous burst noise up to 193 microseconds (2070 symbol epochs) in duration. The convolutionally interleaved data with R-S FEC coding are subsequently trellis coded to ⅔ original code rate and mapped into eight-level digital symbols. The symbols are parsed into 828-symbol sequences.
Annex D specifies that the data frame shall be composed of two data fields, each data field composed of 313 data segments, and each data segment composed of 832 symbols. Annex D specifies that each data segment shall begin with a 4-symbol data-segment-synchronization (DSS) sequence. Annex D specifies that the initial data segment of each data field shall contain a data-field-synchronization (DFS) signal following the 4-symbol DSS sequence therein. The DSS and DFS signals are composed of symbols with +5 or −5 modulation signal values. The 2nd through 313th data segments each conclude with a respective one of the trellis-coded 828-symbol sequences, the convolutional interleaving of which sequences extends to a depth of 52 data segments. The digital symbols are transmitted by eight-level modulation with +7, +5, +3, +1, −1, −3, −5 and −7 modulation signal values. Owing to the A/53 baseband DTV signal being transmitted via vestigial-sideband suppressed-carrier amplitude modulation of a radio-frequency carrier, this eight-level modulation signal is referred to as trellis-coded 8VSB signal. These transmissions are accompanied by a pilot carrier of the same frequency as the suppressed carrier and of an amplitude corresponding to modulation value of +1.25.
The fifth through 515th symbols in the initial data segment of each data field are a specified PN511 sequence—that is, a pseudo-random noise sequence composed of 511 symbols capable of being rendered as +5 or −5 modulation signal values. The 516th through 704th symbols in the initial data segment of each data field are a triple-PN63 sequence. The middle PN63 sequence is inverted in sense of polarity every other data field. The 705th through 728th symbols in the initial data segment of each data field contain a VSB mode code specifying the nature of the vestigial-sideband (VSB) signal being transmitted. The remaining 104 symbols in the initial data segment of each data field are reserved, with the last twelve of these symbols being a precode signal that repeats the last twelve symbols of the data in the last data segment of the previous data field. A/53 specifies such precode signal to implement trellis coding and decoding procedures being able to resume in the second data segment of each field proceeding from where those procedures left off processing the data in the preceding data field.
The 8VSB transmissions have a 10.76 million bits per second baud rate to fit within a 6-megahertz-wide broadcast television channel, and the effective payload is 19.3 million bits per second (Mbps). In an additive-white Gaussian noise (AWGN) channel a perfect receiver will require at least a 14.9 dB signal-to-nose ratio (SNR) in order to keep errors below a threshold-of-visibility (TOV) defined as 1.93 data segment errors per 10,000 data segments, supposing 8VSB signals are broadcast.
After the “ATSC Digital Television Standard” was established in 1995, reception of terrestrial broadcast DTV signals proved to be problematic, particularly if indoor antennas were used. In early 2000 ATSC made an industry-wide call for experts in terrestrial broadcast transmission and reception to join a Task Force on RF System Performance for studying problems with adequate reception and suggesting possible solutions to those problems. By the end of 2000 or so there was general consensus that, besides problems with equalization of the reception channel, there was a need to make the 8VSB signal more robust, if it were to be successfully received during noisy reception conditions. On 26 Jan. 2001 the ATSC Specialist Group on RF Transmission (T3/S9) issued a “Request for Proposal for Potential Revisions to ATSC Standards in the Area of Transmission Specifications”. This RFP concerning how to improve the performance of 8VSB was directed to the DTV industry, universities and other parties interested in the problem. The compatible improvement of fixed and indoor 8-VSB terrestrial DTV service is specified in the widely distributed ATSC RFP to be of top priority.
Subsequent proposals for making the 8VSB signal more robust by altering modulation of the carrier wave share a common problem that the information transmitted in the robust format cannot be utilized by so-called “legacy” DTV receivers that have already been sold to the receiving public. Every 187 bytes of robust payload displace at least 374 bytes of normal payload that can be received by legacy DTV receivers and could be used for HDTV. That is, the amount of information contained in one data segment transmitted by 8VSB as specified by Annex D of the A/53 standard occupies two or more data segments of the robust signal in the proposals for making the 8VSB signal more robust by halving code rate. This means that, if legacy DTV receivers are still to be accommodated with regard to receiving a television program with good resolution in its picture content and reasonably high fidelity in the accompanying sound content, very little payload can be transmitted in the robust format. The problem is particularly vexatious if a part of the normal payload is to be transmitted in the robust format, because most of the proposals for transmission of data in a more robust format have the following requirement. The information content of the part of the normal payload has to be transmitted, not only in the more robust 8VSB format, but additionally in normal 8VSB format so that legacy receivers can be accommodated.
The specification and drawing of U.S. Pat. No. 6,430,159 titled “Forward Error Correction at MPEG-2 Transport Stream Layer” and issued Aug. 6, 2002 to Xiang Wan and Marc H. Morin are incorporated herein by reference. U.S. Pat. No. 6,430,159 describes an error correction operation being performed on a super group of packets within a Transport Stream (TS) using MPEG-2 TS protocol. The forward-error-correction (FEC) coding is formatted as a trailer group of MPEG-2 compliant TS packets containing no payload data, but only an adaptation field. The trailer group packets are provided with PIDs that cause them to be discarded by a standard MPEG-2 decoder. However, an especially equipped MPEG-2 decoder recognizes the PIDs and extracts the trailer group packets to be used for recovering data lost or corrupted in the transmission of the TS. A general concept that can be extracted from U.S. Pat. No. 6,430,159 is that FEC coding can be contained in data packets that do not contain payload data and that are separate from data packets that do contain payload. This concept, as applied to MPEG-2-compliant data packets, was critical to the objectives of the U.S. Pat. No. 6,430,159 invention. Wan and Morin sought to provide a system and method to correct an MPEG-2 transport stream that could be used in any one of the digital video broadcast (DVB) formats, without the need for FEC decoders which were specific to the particular DVB format. Another objective of the U.S. Pat. No. 6,430,159 invention was to avoid appending FEC coding to the end of each packet, in effect adding another layer to the protocol stack. Such a new layer is specific to the transmission architecture and not subject to the MPEG-2 standard. Accordingly, a broadcaster would have to rely upon each intended receiver having a symmetric FEC decoder for the transmitted signal to be received.
The U.S. Pat. No. 6,430,159 invention was not taken up by the satellite broadcast industry, the cablecasting industry or the terrestrial broadcast industry. These industries continued the practice of inserting the original transport stream into a forward-error-correction encoder and broadcasting the resulting signal over their respective broadcast medium to receivers, each having a symmetric FEC decoder for the transmitted signal. The various receivers for satellite broadcast, cablecasting and terrestrial broadcast systems continued to recover MPEG-2-compliant transport streams from received signals, using FEC decoders specific to the various systems and symmetric with the FEC coders employed in these various systems. Wan and Morin had sought to avoid the need for such practices with their U.S. Pat. No. 6,430,159 invention.
Transmitting FEC coding in data packets that do not contain payload data is a practical modification of the current United States standard for digital television broadcasting even though system-specific FEC encoding of the data to be broadcast is appended to each MPEG-2 data packet. Appending system-specific FEC encoding to the MPEG-2 data packets is a practice of the type that Wan and Morin sought to avoid by using their U.S. Pat. No. 6,430,159 invention. Even so, DTV data packets that contain supplemental FEC coding, but do not contain payload data, can be used as the basis for more robust reception of conventional DTV data packets that do contain payload. Such supplemental FEC coding does not affect data segments that contain payload data. This avoids having to transmit the same DTV information twice, once for legacy DTV receivers and again for DTV receivers of new design. The conventional DTV data packets that contain payload are usable by legacy receivers, as well as being part of the robust transmission.
U.S. Pat. No. 6,430,159 describes transverse Reed-Solomon forward-error correction coding being used to generate the adaptation fields that accompany the data fields in a transmission. Transverse Reed-Solomon forward-error-correction codes are applied to paths that cross each of a group of data packets or data segments, with each byte of each data segment being included in one of the paths. The transverse R-S FEC codes generate parity bytes. U.S. Pat. No. 6,430,159 describes such parity bytes being arranged in further packets separate from the data packets. These further packets are similar in general format to data packets and are transmitted according to protocols similar to those used for transmitting the data packets. U.S. Pat. No. 6,430,159 does not convey to one of ordinary skill in the DTV art a full appreciation of the flexibility in transmission system design afforded by such coding, however.
A wide variety of transverse Reed-Solomon codes can be used for providing additional forward-error correction coding for data fields as defined in A/53. A variety of transverse R-S codes can be used for providing additional forward-error-correction coding for a prescribed number of A/53-compliant data segments selected from one or more data fields as defined in A/53. Transverse R-S coding affords greater flexibility in choosing the amount of redundancy in robust transmissions than is provided by proposals for making the 8VSB signal more robust by altering modulation of the carrier wave to halve code rate or to quarter code rate. Broadcasters who participated in the Task Force on RF System Performance expressed a desire for flexibility in reducing code rate and hoped for smaller reductions in coding rate.
Transverse R-S FEC coding facilitates choosing the amount of redundancy in robust transmissions to be larger than the amount of redundancy in normal 8VSB transmissions by factors between one and two. Coupled with not having to transmit the same DTV information twice, once for legacy DTV receivers and again for DTV receivers of new design, this permits DTV transmission to be made more robust while still maintaining higher than standard DTV resolution. This allows DTV receivers of new design to receive HDTV broadcasting of given effective radiated power (ERP) at substantially more reception sites than legacy DTV receivers could receive normal HDTV transmissions from the same transmitter. At the same time, legacy DTV receivers can continue to receive the HDTV broadcasting at the reception sites where those receivers were able to receive normal HDTV transmissions from the same transmitter. Previous proposals for making the 8VSB signal more robust by altering modulation of the carrier wave do not allow robust transmission of HDTV signals receivable by legacy DTV receivers as well as by DTV receivers of new design.
Transverse R-S FEC coding combines with the lateral (207, 187) R-S FEC coding prescribed by A/53 to provide two-dimensional R-S FEC coding. As previously noted, U.S. Pat. No. 6,430,159 disparages the use of lateral R-S FEC coding and consequently teaches away from two-dimensional R-S FEC coding. Two-dimensional R-S FEC coding is known per se in arts other than the DTV art. The recording of digital audio on compact disk uses cross-interleaved Reed-Solomon coding (CIRC). Two-dimensional R-S FEC coding has also been used when recording digital information on magnetic tape.
However, the reasons that two-dimensional R-S FEC coding is advantageous are different in the DTV art than in other arts. Transverse R-S FEC coding is used in modification of the A/53 DTV standard because additional forward-error-correction coding can be introduced into the DTV signal with minimal effect on reception by legacy DTV receivers. The lateral (207, 187) R-S FEC coding prescribed by A/53 provides a means for the transport stream demultiplexer in a DTV receiver to determine whether or not a received data packet contains uncorrected byte errors. So, lateral (207, 187) R-S FEC coding is indispensable when modifying the A/53 DTV standard, particularly to legacy DTV receivers. Accordingly, lateral (207, 187) R-S error correction is performed on data packets subsequent to transverse R-S error correction, as a final step before de-randomization of bits in each data packet. Performing lateral R-S error correction subsequent to transverse R-S error correction reverses the order of two-dimensional R-S error correction that is conventionally used in playback apparatus for magnetic-tape recordings of digital data.
Transverse Reed-Solomon forward-error-correction coding provides little particular assistance to improving equalization. Accordingly, further aspects of the invention concern the time-division-multiplexing of “super-robust” signals into the DTV signal. These super-robust signals use only one-half of the full alphabet of 8VSB symbols, so four rather than eight modulation levels are used. Because broadcast DTV uses trellis coding following convolutional interleaving, rather than block interleaving, the robust-modulation symbols should be ones that can be incorporated into the data stream also including trellis-coded 8VSB symbols without affecting the trellis coding of the 8VSB symbols.
For example, a set of restricted-alphabet 8VSB symbols that map data into just +7, +5, −5 and −7 modulation signal values was proposed by Philips Research. This restricted-alphabet signal is referred to as “pseudo-2VSB”, since the information in the resulting modulation signal is conveyed entirely by the polarity of that signal. Using pseudo-2VSB throughout the entire DTV broadcast would halve the effective payload to 9.64 million bits per second (Mbps), but this is more than sufficient to transmit a standard-definition television (SDTV) signal. The gap between the least negative normalized modulation level, −5, and the least positive normalized modulation level, +5, is 10. This is five times the gap of 2 between adjacent modulation levels in an 8VSB signal, permitting TOV to be achieved at significantly worse SNR under AWGN conditions. The SNR required in order to keep errors below TOV in an AWGN channel is reduced to 8.5 dB, a reduction of 6.4 dB.
That is, about a quarter as much power would be required for satisfactory reception of an AWGN channel, presuming that modulation levels did not have to be decreased to maintain average effective radiated power (ERP) levels within current specification. The average ERP of the pseudo-2VSB symbols tends to increase respective to conventional trellis-coded 8VSB, because of just the +7, +5, −5 and −7 modulation signal values being used and the +3, +1, −1 and −3 modulation signal values of 8VSB not being used. A 1.5 dB decrease in transmitter peak power is necessary if long sequences of modified-2VSB symbols are transmitted. So, if long sequences of pseudo-2VSB symbols are transmitted, the increase in service area for the pseudo-2VSB signal is only that which could be achieved with a 4.9 dB increase in the power of a conventional trellis-coded 8VSB signal. Furthermore, service area for the conventional trellis-coded 8VSB signal accompanying the pseudo-2VSB signal is diminished. Consequently, pseudo-2VSB signals would in actual broadcast practice probably be restricted to only a small number of the data segments in each 313-segment data field.
Various restricted alphabets of 8VSB symbols can be analyzed to determine how the original data transport stream can be modified to cause each of the restricted-alphabet 8VSB symbol streams to be generated during the trellis coding procedure at the transmitter. Each bit in a stream of randomized data can be immediately repeated to generate a modified stream of data supplied to the (207, 187) R-S FEC encoder, which causes a pseudo-2VSB signal to be generated by the trellis coding procedure.
In another, different procedure a ONE is inserted after each bit in a stream of randomized data to generate a modified stream of data supplied to the (207, 187) R-S FEC encoder. This modified stream of data causes the trellis coding procedure to generate a restricted-alphabet signal which excludes the −7, −5, +1 and +3 symbol values of the full 8VSB alphabet. Pilot carrier energy is increased substantially in the resulting modulation, which makes synchrononous demodulation easier in the DTV receiver. The gap between the least negative normalized modulation level, −5, and the least positive normalized modulation level, +1, is 6 in this restricted-alphabet signal. This gap is thrice the gap of 2 between adjacent modulation levels in an 8VSB signal, permitting TOV to be achieved at significantly poorer SNR under AWGN conditions than is the case with 8VSB signal or with E-4VSB signal. Better SNR under AWGN conditions is required to achieve TOV than is the case with pseudo-2VSB. This restricted-alphabet signal has substantially less average power than a pseudo-2VSB signal, but somewhat higher average power than normal 8VSB signal.
In still another, different procedure a ZERO is inserted after each bit in a stream of randomized data to generate a modified stream of data supplied to the (207, 187) R-S FEC encoder. This modified stream of data causes the trellis coding procedure to generate a restricted-alphabet signal which excludes the −3, −1, +5 and +7 symbol values of the full 8VSB alphabet. The gap between the least negative normalized modulation level, −5, and the least positive normalized modulation level, +1, is also 6 in this restricted-alphabet signal. However, this restricted-alphabet signal has somewhat less average power than normal 8VSB signal. A difficult problem with using just this restricted-alphabet signal is that the polarity of the pilot signal is reversed in the resulting modulation, which interferes with synchronous demodulation in DTV receivers, particularly legacy ones.
A receiver for broadcast DTV signals can use different symbol decoding procedures depending on whether the full alphabet of 8VSB symbols is being transmitted thereto or only half of the alphabet of 8VSB symbols is being transmitted thereto. If symbol decoding is done by Viterbi trellis decoding procedures, the decoding tree can be pruned to exclude decoding possibilities that are ruled out by knowledge that only half of the alphabet of the 8VSB symbols was transmitted. However, this presumes that at the time that symbol decoding is done, the DTV receiver has knowledge available to it as to whether the currently received DTV signal was transmitted using the full alphabet of 8VSB symbols or only half of that alphabet. The packet identification (PID) code bits of a data packet indicates whether the full alphabet of 8VSB symbols or only half of that alphabet was used in generating the data packet, but that information is not timely available at the receiver. The convolutional byte interleaving done at the transmitter before forward-error-correction coding breaks the PID into two parts and disperses the parts within the data field. The convolutional de-interleaving done in the receiver subsequent to symbol decoding restores the PID but only a considerable time after completion of the symbol decoding of the bytes including the PID.
Information as to whether the currently received DTV signal was transmitted using the full alphabet of 8VSB symbols or using only half of that alphabet can be transmitted in coded form during the 92-symbol “reserved” portion of the initial, zeroeth data segment of a data field. This “reserved” portion immediately follows the data field synchronization (DFS) signal. On 19 Dec. 2002 Philips Research described a general concept for doing this, as part of a proposal to ATSC for enhancing 8VSB signals. On Apr. 9, 2002 V. R. Gaddam and D. Birru filed U.S. patent application Ser. No. 118,876 titled “Packet Identification Mechanism at the Transmitter and Receiver for an Enhanced ATSC 8-VSB System”. This application assigned to Koninklijke Philips Electronics N.V. was published Dec. 19, 2002 with publication No. 20020191712. This publication describes the pattern of data segments for normal transmission and for robust transmission in a data field yet to be convolutionally interleaved being inserted into a bit map convolutionally interleaved to provide a homologue of the byte map of interleaved data that is trellis coded.
On Dec. 3, 2001 M. Fimoff, R. W. Citta and J. Xia filed U.S. patent application Ser. No. 011,333 titled “Kerdock Coding and Decoding System for Map Data”. This application assigned to Zenith Electronics Corporation was published Mar. 27, 2003 with publication No. 20030058140. This publication describes Kerdock codes that code different patterns of robust transmission within data fields being inserted into the initial, zeroeth data segment of each data field. This method can be adapted for describing DTV signal transmitted using the full alphabet of 8VSB symbols or using only half of that alphabet.